How deep does an outside-air intake plenum box have to be for 100,000 CFM?

Question

From a louver of 400 sq.ft with 50% free area.

Answer 1

I couldn't find any helpful information on the web. I'd think you would want sufficient depth so the flow through the plenum to the duct doesn't encounter any more resistance than it does in the duct. This suggests a rule of thumb. Visualize the flow in the plenum as converging on the duct. The minimum area it flows through occurs right at the boundary of the duct, and is equal to the perimeter of the duct times the plenum depth. Then you would want a depth D such that:
For a round duct of radius R, 2*pi*R*D >= pi*R^2, or D >= R/2.
For a rectangular duct of width A and height B, 2*(A+B)*D >= A*B, or D >= A*B/(2*(A+B)).
I hope this helps.